drm/i915: Don't set persistent FBC mode on ILK/SNB

The ILK/SNB docs are a bit unclear what the persistent mode does, but
the CTG docs clearly state that it was meant to be used when we're
tracking back buffer modifications. We never do that, so leave it in
non-persistent mode.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -230,8 +230,6 @@ static void ironlake_enable_fbc(struct drm_crtc *crtc)
 	dpfc_ctl = I915_READ(ILK_DPFC_CONTROL);
 	dpfc_ctl &= DPFC_RESERVED;
 	dpfc_ctl |= (plane | DPFC_CTL_LIMIT_1X);
-	/* Set persistent mode for front-buffer rendering, ala X. */
-	dpfc_ctl |= DPFC_CTL_PERSISTENT_MODE;
 	dpfc_ctl |= DPFC_CTL_FENCE_EN;
 	if (IS_GEN5(dev))
 		dpfc_ctl |= obj->fence_reg;